1. Field of the Invention
This invention relates to a digital display clock for motor vehicles and in particular to digital type clock for motor vehicles which utilize a fluorescent type tube as the time display.
2. Description of the Prior Art
Since one must be able to read the time quickly and easily while driving a motor vehicle, a digital type display clock finds particular utility. Among the various elements available for the digital display, such as luminescent diodes, liquid crystals, fluorescent display tubes, etc., the fluorescent display tube has very excellent characteristics such as strong brightness for easy reading, low voltage operation, a greenish-blue display which is easy to read, freedom of choice of letter size, long life and high mechanical strength.
In a motor vehicle environment, an important requirement for the digital display clock is that its brightness must be adjustable according to the level of the ambient light. A digital display clock utilized in a motor vehicle is used at various times of the day under a variety of light ambient conditions which depend upon the weather and the season. Furthermore, the display must be easily read under all of the conditions. In other words, a time display which is hardly readable under strong daylight conditions but perfectly readable in darkness at night is insufficient and conversely a bright display which is easily read in daylight is dazzling at night and also insufficient. Furthermore, since the motor vehicle is subjected to the direct outdoor light and darkness, the level of the brightness of the clock display is even more important. In particular, a dazzling time display at night may adversely affect the driver's visibility, and therefore maneuverability. Consequently, the brightness of the clock display must be freely adjustable.
A fluorescent type display tube is ideal for motor vehicle clocks because the light intensity can be easily adjusted by changing its anode voltage, grid voltage or duty cycle. In motor vehicle clocks with digital displays using fluorescent tubes, a circuit for adjustment of the anode voltage has been provided in a clock system of the prior art. Such a prior art system suffers from several defects. First of all, such a circuit for adjustment of the brightness of the fluorescent tube is very complex. Furthermore, the prior art circuit in which the display circuit and the circuit for brightness control are combined into one, the light control circuit must be altered whenever a change is made in the display control circuit.